1. Field of the Invention
The present invention relates to a soft magnetic thin film for use in a magnetic head for recording information on and reproducing information from a magnetic recording medium in an 8-mm VTR or a hard disk drive.
2. Description of the Prior Art
To meet demands for high-density magnetic recording with higher track densities and shorter wavelengths, it is necessary for magnetic heads to have soft magnetic thin films that are made of materials having high saturation flux density and high permeability.
Such soft magnetic thin film materials should preferably be amorphous alloys which can be manufactured by quenching a molten liquid and deposition from a vapor phase. Since amorphous alloys do not match the present magnetic head fabrication process due to lack of sufficient heat resistance, however, crystalline alloy materials have heretofore been used as soft magnetic thin film materials.
One material characteristic which is deeply involved in the reproduction characteristics of magnetic heads is permeability. The permeability is determined by how fast a spontaneous magnetic moment of ferromagnetic material responds to or follows an external magnetic field. If the spontaneous magnetic moment quickly follows a small leakage magnetic field from a recording medium, then the material has a high permeability, and the magnetic head has a high reproduction efficiency.
If the soft magnetic thin film of a magnetic head has such a property that its magnetic moment tends to be oriented only in a certain direction, i.e., if the soft magnetic thin film is magnetically anisotropic, then its spontaneous magnetic moment is prevented from following the direction of an external magnetic field, and the permeability of the soft magnetic thin film is low. Therefore, the soft magnetic thin films are required to have their magnetic anisotropy minimized.
Assuming that the angle between the spontaneous magnetization in the plane of a soft magnetic thin film and a reference direction is indicated by .theta., the total magnetization energy Ktot(.theta.) at the angle .theta. is given as the sum of the magnetization energy due to the crystalline magnetic anisotropy resulting from the crystalline structure of the soft magnetic material at the angle .theta., i.e., the crystalline magnetic anisotropy magnetization energy Kc(.theta.), the magnetization energy Kst(.theta.) due to the stress-induced magnetic anisotropy resulting from the exertion of a stress, and the magnetization energy Ksh(.theta.) due to the shape magnetic anisotropy depending on the shape of the crystal of the soft magnetic thin film, as indicated by the following equation (1): EQU Ktot(.theta.)=Kc(.theta.)+Kst(.theta.)+Ksh(.theta.) (1).
When the total magnetization energy Ktot (.theta.) is plotted with respect to the angle .theta., it is represented by a curve having maximum and minimum values as shown in FIG. 1 of the accompanying drawings. As the difference .vertline.Ktot.vertline. between the maximum and minimum values of the curve, i.e., the maximum amplitude of the curve, is smaller, the permeability is higher and the coercive force is lower, making the soft magnetic thin film preferable for use in a magnetic head. The difference .vertline.Ktot.vertline. will hereinafter referred to as a total magnetic anisotropy energy value. When the crystalline magnetic anisotropy magnetization energy Kc(.theta.), which can have both positive and negative values, is plotted with respect to the angle .theta., it is represented by a curve as shown in FIG. 2 of the accompanying drawings. The difference between the maximum and minimum values of the curve in FIG. 2 is represented by .vertline.Kc.vertline., and will hereinafter referred to as a crystalline magnetic anisotropy energy value.
Generally, in the manufacture of a soft magnetic thin film with a small total magnetic anisotropy energy value .vertline.Ktot.vertline., since the shape magnetic anisotropy cannot directly be measured, the composition of materials is selected such that the crystalline magnetic anisotropy and the stress-induced magnetic anisotropy will be as small as possible without taking the shape magnetic anisotropy into account.
In the case where a soft magnetic thin film is made of an alloy, the crystalline magnetic anisotropy thereof is given as the product of a constant (which is a quantity called K.sub.1 in the unit of (J/m.sup.3) if the alloy is of a cubic system) determined by the composition of the alloy and a factor depending on the orientation distribution function of the specimen. The composition of the alloy is selected such that .vertline.K.sub.1 .vertline. will be smaller. The stress-induced magnetic anisotropy is proportional to the product .lambda..times..sigma. where .lambda. is the magnetorestriction and .sigma. is the stress. If the soft magnetic thin film is formed by sputtering, then the stress .sigma. is frequently of a large value. Even if conditions are established to suppress the stress .sigma., it is difficult to reduce the stress .sigma. with high reproducibility owing to a slight deviation of the conditions. Therefore, the composition of the alloy is selected such that the magnetorestriction .lambda. depending on the alloy composition will be smaller.
As described above, in order to reduce the total magnetic anisotropy energy value .vertline.Ktot.vertline. of a soft magnetic thin film, it has heretofore been customary to select a composition with a smaller magnetorestriction .lambda. and a smaller crystalline magnetic anisotropy constant K.sub.1. Alloy materials that satisfy such conditions are practically only an FeAlSi alloy, i.e., Sendust, and an FeRuGaSi alloy (Sofmax: commercial name).
However, even when a soft magnetic thin film is made of an FeAlSi alloy or an FeRuGaSi alloy whose crystalline magnetic anisotropy constant K.sub.1 and magnetorestriction .lambda. are 0, the total magnetic anisotropy energy value .vertline.Ktot.vertline. is not sufficiently small. This is because the shape magnetic anisotropy, which has not been taken into account heretofore, i.e., the third term Ksh (.theta.) of the equation (1), is a large factor.
Specifically, even when only the crystalline magnetic anisotropy energy value .vertline.Kc.vertline. is reduced in an attempt to reduce the total magnetic anisotropy energy value .vertline.Ktot.vertline., the total magnetic anisotropy energy value .vertline.Ktot.vertline. is not necessarily reduced under the influence of a large shape magnetic anisotropy that is produced by the columnar crystalline structure of the soft magnetic thin film.
Soft magnetic thin films for use in magnetic heads are fabricated by depositing an alloy, used as a target, on a substrate by sputtering or vacuum evaporation.
According to one fabrication process, metallic particles are directed perpendicularly to the substrate surface so as to be deposited thereon. However, in the manufacture of recently popular magnetic heads with magnetic gap surfaces covered with soft magnetic thin films, metallic particles are directed and deposited on a thin-film-forming surface at an angle to the direction normal to the thin-film-forming surface. Such a practice is referred to as oblique deposition or tilted substrate orientation.
A soft magnetic thin film made of a crystalline alloy that is obliquely deposited by sputtering or vacuum evaporation has its total magnetic anisotropy energy value .vertline.Ktot.vertline. largely increased by the shape magnetic anisotropy.
The reasons for such a large increase in the total magnetic anisotropy energy value .vertline.Ktot.vertline. are as follows: The materials of the soft magnetic thin film that is obliquely deposited are not uniformly packed, but have gaps in the crystal boundary and are of a columnar structure. When the soft magnetic material with gaps which is composed of columns separated by nonmagnetic regions such as oxide films is magnetized, it is easily magnetized in its longitudinal direction. Therefore, a large shape magnetic anisotropy occurs due to the shape (e.g., rod or plate shape) of the columnar crystal and the angle at which the columnar crystal is tilted.
When a soft magnetic thin film is formed by oblique deposition, the stress .sigma. developed in the thin film may sometimes be extremely large. If an extremely large stress .sigma. is developed, then it causes a large stress-induced magnetic anisotropy .vertline.Kst.vertline. to be produced by combination with a slight magnetorestriction .lambda., with the result that the total magnetic anisotropy energy value .vertline.Ktot.vertline. is not reduced.